The present invention relates generally to boost circuits for use in low-power memory devices, and more particularly, to a word line boost circuit that is independent of supply voltage.
For low voltage operation of a memory device, for example a xe2x80x9cFlashxe2x80x9d memory device, word line-voltage-boosting is used to boost the voltage levels of word line signals used to operate the memory.
FIG. 1 shows a diagram of a typical boost circuit used in a memory device. The boost circuit 100 receives a supply voltage (Vcc) and a boost control signal 102 to produce a boosted signal (VPXG) for use by the memory device. For example, the VPXG signal is a global programming pulse that is input to sector select circuits 104. The sector select circuits 104 receive sector control signals 106 that activate a selected sector select circuit to produce a sector programming pulse 108 derived from the VPXG signal. The sector programming pulses 108 are received by Xdecoder circuits 110 that output boosted word line signals 112 in response to received decode signals 114. The boosted word line signals are based on the sector pulses 108.
However, typical boosting circuits, such as the boost circuit 100, produce VPXG signals that are dependent on the supply voltage (Vcc). As a result, the boosted VPXG signal may have voltage variations that cause the derived word line signals to fail meeting high and low voltage margins required to operate the memory device.
FIG. 2 shows a graph illustrating a dependence relationship between core cell current (I) and a voltage derived from the core cell current in a conventional boost circuit. For example, a boosted word line voltage results in a core cell current that is converted to a voltage. The indicator at Iref indicates a current level used as a reference to compare to a core cell current to determine a data value in a memory device. The indicator I1 indicates a desired current level to be read from the core cell to determine a data value of 1, and the indicator I0 indicates a desired current level to be read from the core cell to determine a data value of 0.
If the word line voltage is dependent on supply voltage, the core cell current created by the varying word line voltage may degrade current margins such that it becomes difficult to read the core cell data value. For example, if the word line voltage is too low or too high, the current margin to read a data value of 1 or 0 will become smaller. Thus, if the boosted word line has a power supply dependence, power supply variation may result in degraded memory operation.
Therefore is would be desirable to have a booster circuit that operates independently from supply voltage to produce boosted signals in a memory device.
The present invention includes a system-that provides for word line boosting that is independent of supply voltage. The system includes a pre-charge stage to pre-charge a capacitive boost element, and a boost stage to produce the boosted memory signal. Thus, the boosted memory signal is boosted independently of the supply voltage (Vcc).
In one embodiment of the invention, a circuit for providing a boosted signal used to produce a word line signal in a memory device is provided. The circuit includes a precharge stage that has an output terminal and is coupled to receive an address signal and a boost control signal. The pre-charge stage operable to produce the boosted signal having a pre-charged level at the output terminal. The circuit also includes a capacitive element that has a first and a second terminal. The first terminal of the capacitive element is coupled to the output terminal. A boost stage is included that is coupled to receive the boost control signal and produce a boost activation signal at a boost stage output terminal that is coupled to the second terminal of the capacitive element. When the boost activation signal is active, the boosted signal is set to a selected boost level that is independent of supply voltage.
In another embodiment of the invention, a method for providing a boosted signal used to produce a word line signal in a memory device is provided. The method includes steps of receiving an address signal, pre-charging the boosted signal to a pre-charge level, receiving a boost control signal, and boosting the boosted signal from the pre-charge level to a boosted level that is independent from a supply voltage.